Tom of T.H.U.M.B. is an agent of, well, T.H.U.M.B.
We never learn Tom's last name so the only way to specifically single him out is to add the acronym for the agency he works for.
'Works for' is itself saying a bit much since he is the only agent in the organization with the exception of his sidekick, Swinging Jack.
"Tom, an average member of U.S. Intelligence maintenance department. Wounded in the line of duty, Tom and his faithful assistant, Swinging Jack, ran afoul of the shrinking laser beam ray gun. They gave their height for their country, thus creating the Tiny Human Underground Military Bureau. When the plan calls for small thinking, it's a job for Tom of T.H.U.M.B."
The above is the introduction we get on each of the televised animated adventures which exist of Tom and Jack's missions. The 'wounded in the line of duty' makes it sound more impressive that it really is. Tom was sweeping at the top of the stairs, fell down those steps, and landed near a large machine. When his assistant, Smiling Jack, helps him stand, Tom's arm accidentally hits a lever on that device, moving in to the on position which opened a compartment on the wall from where a laser cannon slid out and hit both in its light.
That beam shrunk the two to around 3" tall. I say around because one reference has it 4" and another implied 2". The miniaturization is apparently permanent because after the event, Tom moved his domicile into a middle drawer of a file cabinet, said drawer labeled with the new 'agency' acronym. The compartment holds a tiny bed and an easy chair and lamp and little else, each of these items apparently also resized with the ray. [No mention is made of where Swinging Jack resides.]
Other items which likely were reduced to Tom's dimensions include a two-man helicopter and a jet fighter plane, as well as smaller items like his weapons.
To say that Tom was a particularly bad agent who succeeds more out of just plain dumb luck is putting it mildly. He has the best of intentions but things always seem to go wrong yet he gets good results eventually. One of the reasons things misfire is the truly inept way Swinging Jack goes about, well, everything. So, whether Tom is really bad or just has bad help is debatable.
All good agents need a good nemesis and Tom has that in the evil scientists of M.A.D. which stands for "Maladjusted, Anti-social, and Darn mean". These fellows and their henchmen, inevitably donned in black hats and black trenchcoats, are "bent on destroying the world for their own gains". [What they gain when they blow up the world, which they try to do a couple of times, is never explained.]
The hefty, thick-mustached man who has the dubious joy of being Tom's boss at U.S.Intelligence is Chief Homer J. Chief. It is hard to say whether this man is grumpy all the time because he has a lousy disposition or because he really hates having to call in Tom for the occasional assignment is never revealed. The fact that his last name is 'Chief' and his title is 'Chief' may add to his mood.
If Tom ever noticed anything, though, he never shows it. Tom is the most eager go-getter you could want in an agent and never shows fear or doubt. Intrepid seems to be Tom's motto.
Tom has a couple of catch-phrases he uses on occasion:
"Can't win them all, Chief Chief"
"On the ball and ready for action!"